1. Field of the Invention
The present invention relates to a fixing device for conveying and at the same time heating a sheet, and in particular, to an image forming apparatus, such as a laser printer, a digital copier, a plain paper facsimile, etc., including the fixing device.
2. Discussion of the Background Art
Conventionally, there is provided a fixing device in an image forming apparatus to fix a toner image visualized by toner onto a printing medium, such as a transfer sheet, etc. In such a fixing device, the toner image is fixed due to melting of the toner when the toner image passes through a nip created between a fixing roller or belt heated and rotated by a prescribed device and a pressure roller or belt pressing against the fixing roller or the like.
Since it consists mainly of resin, the toner is fused at the pressing section and sticks to the fixing roller or the like. Accordingly, various devices have been proposed to suppress such toner sticking, for example, by adding wax to the toner, or by coating releasing agent, such as silicon oil, etc., to the surface of the fixing roller.
Further, a sheet separation mechanism having a separation plate is arranged adjacent to the fixing roller or belt to forcibly separate a sheet winding around the fixing roller or belt due to melting of the toner.
However, since the separation plate slidingly contacts the fixing member, the toner readily accumulates at the contact section, and sometimes drops and contaminates the printing medium. Further, a slide contact mark is put on the fixing member for the same reason, so that its service life decreases and an abnormal image is sometimes formed on the printing medium.
However, due to difficulty in handling the releasing agent in the fixing device, wax is often used instead and is added to the toner.
Thus, the above-mentioned problem becomes aggravated, and accordingly, various inventions related to the rotational member and the separation belt have been proposed.
At the same time, to maintain the same or between separation performance as the separation plate without contacting the rotation fixing member, a gap between a leading end of the separation mechanism and the rotational fixing member needs to be as small as possible, in units on the order of 0.1 mm. That is, the leading end needs to be positioned as closer as possible to the exit of the nip.
However, when arranging the leading end of the separation plate as mentioned above, fine vibration of the surface of the rotational fixing member and an extra ordinarily narrow space located immediately downstream of the nip can cause problems. For example, as described in Japanese Patent Application Laid Open No. 2004-093582, when a non-contact separation plate is employed in a fixing belt system, the gap cannot be precisely adjusted due to floating or rippling of the fixing belt at the exit of the nip.
To suppress the floating or fluctuation of the fixing belt, a driving roller is provided to contact the outer circumference of the fixing belt. Alternatively, a substrate of the fixing belt is thinned or a tension roller is provided inside the fixing belt.
However, even with such countermeasures, the separation plate cannot track fluctuation of the fixing belt because the separation plate does not contact the fixing belt. Further, due to unevenness of manufacturing precision of parts, there is a limit on how finely the gap can be adjusted. More over, even though the fluctuation of the fixing belt and that of the gap can be suppressed in the initial stage of usage, an elastic layer of the fixing roller softens and/or deforms. As a result, the gap fluctuation becomes hardly suppressed as time elapses.
When the elastic layer of the fixing roller softens as the pressure roller contacts the surface of the fixing belt, the pressure roller bites into the fixing belt more than initially, thus increasing the gap. When the tension roller is arranged inside the fixing belt and the elastic layer of the fixing roller softens, the driving roller draws the fixing belt more than initially, and the gap decreases. As a result, in the worst case, the separation plate interferes with and damages the fixing belt.
Further, it is proposed in Japanese Patent Application Laid Open No. 2006-243471 that a separation member is arranged inside a fixing belt as a separation device immediately downstream of the fixing nip to change its separation curvature and improve a separation performance of the fixing belt. However, the separation member is arranged as in Japanese Patent Application Laid Open No. 2004-093582, and accordingly, the gap needs to be additionally adjusted as time elapses.
Further, a thin coat sheet is increasingly demanded in a market recently, and possibly causes the above-mentioned separation problem.
Further, the leading end of the separation plate of such an apparatus cannot accurately track fluctuation of the fixing belt arranged adjacent thereto. As a result, the leading end sometimes damages the surface of the fixing belt or the gap between the leading end of the separation plate and the surface of the rotational fixing member cannot be decreased beyond a prescribed level.
Further, to increase a biting amount of a fixing roller into a pressure roller and accordingly a nip width therebetween for the purpose of maintaining the fixing performance in the narrow space, a sponge member made of foam silicon or the like has come to be employed as an elastic layer of the fixing roller. As a result, although such an arrangement does minimize a warm-up period it also aggravates the above-mentioned problem.